In the recovery of certain distillates from crude oil in fractionation apparatus commonly known as fluid catalytic cracking plants, it is common practice to introduce a catalyst into the reactor employed in the plant in order to accomplish the desired distillation. As a result of the introduction of the catalyst into the reactor, the material used as the catalyst acquires a coating of carbonaceous material as a deposit tightly adhering to the surface thereof.
It has been found economical to remove the surface deposit from the catalyst in a regenerator so that the catalyst may be reintroduced or recycled into the reactor to maintain a continuous distillation cycle or operation. The coating or surface deposit, which is usually referred to as coke, is removed by subjecting the coated catalyst to air in a high temperature atmosphere within a regenerator in the form of a pressure vessel. The high temperature atmosphere comprises high pressure air serving as a fluidizing medium and a source of oxygen for combustion of accumulated surface deposits (coke) on the catalyst. This process is referred to as "burning off" the surface accumulation or coke from the catalyst.
Once the carbonaceous material has been removed from the surface of the catalyst, the catalyst is removed from the regenerator and reintroduced along with the crude oil into the reactor where the distillation process of the crude oil continuously occurs. The hot products of combustion created in the regenerator vessel and containing entrained catalyst particles are withdrawn from the vessel by venting them to the atmosphere through appropriate disposal devices such as a scrubber or the like for removing air contaminents or for recovering energy from the gas flow.
Commonly assigned U.S. Pat. No. 3,855,788 discloses a power recovery system in which a turbine is driven by the gaseous products of combustion from the regenerator vessel. The turbine, in turn, drives a string including an air compressor and a generator. Due to the relatively large volumes of gases created in the regenerator vessel, it is necessary to use relatively large valves in the four to five feet in diameter conduits leading from the vessel. Large valves are relatively slow in responding to manual or automatic signals which call for repositioning of the valve due to flow, pressure and/or temperature changes occurring in the vessel. Thus, undesirable operating conditions may occur before appropriate adjustment of flow control valves can be made and may continue for a period such that operation of the mechanical equipment would be terminated because of shaft overspeed. Control problems are overcome by providing a small diameter (on the order of one and one-half feet) bypass line around the turbine to control flow through the turbine in response to load demand. This arrangement, however, required a compressor to provide a full time load and presented some problems in retrofitting existing fluid catalytic cracking plants.
Commonly assigned U.S. Pat. No. 4,211,932 discloses a power recovery system in which a turbine drives a generator as the only load. The power recovery string serially includes a turbine or expander, a speed reducing mechanism, an induction or synchronous generator and an eddy current brake. Since this arrangement does not include the inherent power absorbing load of a compressor, the coils of the eddy current brake are excited to provide the necessary load during other than steady-state conditions such as when the electrical load of the generator is disconnected.